1. Field of the Invention
The present invention relates to a filter medium that is used in conjunction with concentric tube type strainers used in emergency core cooling systems (ECCS) of nuclear power plants. Specifically, the present invention is a woven or knitted stainless steel filter medium that is inserted between an external and an internal double wall concentric tube type strainer and through which water must pass and be filtered after the water passes through the strainer and before the water can be pumped to the reactor core. This filter medium alters the approach velocity of the water at the strainer surface resulting in non-uniform fibrous debris accumulation on the strainer surface which better insures acceptable strainer debris hydraulic head loss.
2. Description of the Related Art
The containment sump, which is also known as the emergency or recirculation sump, is part of the Emergency Core Cooling System or ECCS of a nuclear power plant. The ECCS is one of several safety systems required by the Nuclear Regulatory Commission or NRC in every nuclear power plant constructed in the United States. The ECCS is required in order to mitigate a design basis accident.
The containment sump collects reactor coolant and chemically reactive spray solutions following a loss-of-coolant-accident or LOCA. The containment sump serves as the water source to support long-term recirculation for the functions of residual heat removal, emergency core cooling, and containment atmosphere cleanup. Thus this water source and related equipment are important safety components of the nuclear power plant.
Typically, the containment sump is surrounded by a debris strainer to prevent debris from entering the ECCS suction lines. The strainer is constructed with a plurality of perforated strainer modules that serve to strain and filter out fibrous and particulate debris and other material from recirculation water that enters the nuclear power plant containment sump during an emergency core cooling event. It is important to filter out this debris from the water before the water passes into the pumps of the ECCS. Debris can block or restrict flow through the openings in the strainer or, if allowed to pass through the strainer and into the pumps of the ECCS, can damage the ECCS pumps and components in the systems served by the ECCS pumps such as the reactor.
The present invention reduces or eliminates debris and other material that potentially passes through the strainers used in nuclear power plant containment sumps. For a typical strainer, debris quantities that can bypass or pass through the openings of the emergency strainers are typically on the order of five percent or more of the total debris and material that deposits onto the strainer surface. Thus a significant amount of debris typically will pass through the strainer. The amount of debris and material passing through the strainer has been shown to have potentially adverse affects on equipment and components downstream of the strainer. This can include excessive wear, plugging, blockage, fouling, etc. This can result in equipment failure, decreased equipment life, clogging of the reactor fuel cooling paths, etc. Hence the purpose of the present invention is to reduce or eliminate the bypassing of debris and material in order to minimize or eliminate adverse affects on downstream equipment and components.
The nuclear power industry has not employed any type of filter medium in association with the strainers that could become clogged because this could result in loss of head pressure, cause loss of suction on the pumps resulting in cavitation, and thereby preventing the flow of cooling water to the reactor. Thus, the addition of filter medium to the strainers goes against current practice in the industry.
The present invention consists of the primary strainer device, which is typically constructed of perforated stainless steel, coupled with a custom designed filter medium insert of stainless steel knitted or woven wire mesh. The drawings that are attached depict a particular perforated strainer design and utilize a cylindrical knitted wire mesh shape filter medium to act as the secondary filter. However, the filtering medium can be produced in various shapes and configurations for use with different strainer configurations.
The knitted or woven wire mesh filter medium is customized by using various diameters, densities, crimping styles, and configurations to optimize the reduction of bypassed debris and other material for each particular application. The knitted or woven wire mesh is located inside of or on the downstream side of the primary strainer and, as such, serves as a secondary filtering medium, with the strainer serving the primary filtering function. The debris straining effect of the primary strainer combines with the secondary filter to produce a system that greatly reduces bypassed debris and other material potentially encountered during recirculation operation of the ECCS and it does so with no significant increase in fluid pressure drop across the combination strainer and filter.
The combination strainer and filter of the present invention produces two physical factors that reduce or eliminate the bypassing of debris and other material. The first is bypass resistance and the second is entrapment of the debris or other material on the filter medium.
Bypass resistance is the physical resistance to debris and other material from passing through openings in the primary strainer. Use of the knitted or woven wire mesh secondary filter medium in combination with the strainer causes an increase in bypass resistance associated with the primary strainer. This bypass resistance factor tends to cause a significant fraction of the debris and other material that may otherwise pass through the primary strainer to remain on the upstream surface of the primary strainer.
For debris and other material that manages to pass through the openings in the primary strainer, the knitted or woven wire mesh secondary filter medium entraps most, if not all, of the remaining debris and other material within the convoluted wire mesh structure, and thus prevents it from passing into the downstream flow of cooling water traveling to the ECCS pumps.
The present invention is designed to function as a totally passive device without requiring electrical power, periodic replacement, or human intervention or operation, other than periodic inspections approximately once every 18 to 24 months. The use of stainless steel material for the filter medium will ensure the present invention has long term functionality, will not be affected by corrosive conditions, and can remain submerged in the containment water conditions expected following a postulated LOCA.
As discussed in U.S. Nuclear Regulatory Commission (NRC) Regulatory Guide 1.82, Revision 3, “Water Sources for Long Term Recirculation Cooling Following a Loss-of-Coolant Accident”, an emergency core cooling recirculation strainer system needs to be designed and evaluated to address a “thin bed” of fibrous debris. It is commonly known from industry testing that this “thin bed” of fibrous debris combined with a large particulate debris load can result in significant clogging and blockage of water flow through the strainer system. A desired feature of the strainer system is for the strainer system to accumulate or “load” fibrous debris in a non-uniform debris pattern. A thin uniform layer of fibrous debris combined with particulate debris can produce significant blockage.
However, if the fibrous debris accumulates thicker in one area and thinner in another area, the blockage of water flow will be less, resulting in less hydraulic head loss through the strainer system.
The present invention enhances the non-uniform fibrous debris accumulation on the surface of the strainer. In addition to the secondary filtering capabilities of the present invention, the knitted woven wire mesh filter provides an internal flow resistance medium within the strainer. The water flow path is first through the strainer surface, then through the filter medium and finally into the general flow stream to the suction of the emergency core cooling system. This flow path, which passes radially through the filter medium along the length of the strainer, produces an increasing pressure drop of the fluid until exiting the strainer into the general flow stream. This increasing pressure drop results in a varied approach velocity of the bulk fluid to the strainer surface such that the approach velocity is higher at the base of the strainer and lower at the end of the strainer. This varied approach velocity of the bulk fluid results in fibrous debris accumulating in a pattern of a thicker debris bed accumulation at the base of the strainer tapering to a thinner debris bed at the end of the strainer. This non-uniform debris accumulation feature enhances the present invention's ability to accommodate “thin bed” fibrous debris loads to better insure acceptable strainer debris hydraulic head loss.